Translucent bodies of PCA are known. Coble, in U.S. Pat. No. 3,026,210 discloses the use of 0.0625 to 0.5 wgt. percent MgO to aid in the elimination of residual porosity by preventing the entrapment of pores inside alumina grains and to control grain growth in the late stages of sintering, when the density exceeds 98%.
Charles, et al., in U.S. Pat. No. 4,285,732, suggests the addition of small amounts of zirconia and/or hafnia to the alumina-magnesia ceramic to control the formation of a second phase, usually, spinel; i.e., magnesium aluminate, MgAl.sub.2 O.sub.4, which can occur when the magnesia content increases above the solubility level. Grain sizes in the sintered bodies are said to be 20 microns to 50 microns. The range of dopants included 0.030-0.150 wgt. percent magnesia and 0.002-0.070 wgt. percent zirconia and/or hafnia.
U.S. Pat. No. 3,377,176 discloses an alumina composition employed in arc tubes and utilizing 0.05 wgt. percent magnesia and 0.035 wgt. percent yttria.
Maekawa et al., in European Pat. Application No. 93-119959.0, disclose a triple doped alumina including 0.010-0.080 wgt. percent magnesia, 0.001-0.030 wgt. percent yttria, and 0.020-0.120 wgt. percent zirconia.